Add a chapter describing mdbook-spec

Author: ehuss

contributor-guide/src/SUMMARY.md

@@ -5,7 +5,7 @@
 - [Publishing process](publishing.md)
 - [Reference tooling]()
     - [Building the Reference](tooling/building.md)
-    - [mdbook-spec]()
+    - [mdbook-spec](tooling/mdbook-spec.md)
 - [Tests]()
 - [Formatting]()
     - [Markdown]()

contributor-guide/src/tooling/mdbook-spec.md

@@ -0,0 +1,23 @@
+# mdbook-spec
+
+[`mdbook-spec`](https://github.com/rust-lang/reference/tree/master/mdbook-spec) is an mdBook preprocessor that adds features to the Reference. It provides:
+
+- Parsing and generation of [grammar diagrams](../grammar.md).
+    - [Automatic grammar production links](../grammar.md#automatic-linking).
+    - Generation of the [grammar summary appendix](https://doc.rust-lang.org/nightly/reference/grammar.html).
+- [Automatic standard library links](../links.md#standard-library-links).
+- Handling of [rule names](../rules.md).
+    - Validation of the names.
+    - Converting rule names to links.
+    - [Automatic rule link references](../links.md#rule-links).
+    - Generation of [links to rule tests](../rules/test-annotations.md).
+    - Generation of the [test summary](https://doc.rust-lang.org/nightly/reference/test-summary.html).
+- Support for [admonitions](../formatting/admonitions.md).
+
+## Environment variables
+
+There are a few environment variables that `mdbook-spec` uses, described in the [Building chapter](building.md):
+
+- [`SPEC_RELATIVE`](building.md#SPEC_RELATIVE) --- Can be set to link external books to the live site.
+- [`SPEC_DENY_WARNINGS`](building.md#SPEC_DENY_WARNINGS) --- Whether warnings should be treated as errors.
+- [`SPEC_RUST_ROOT`](building.md#SPEC_RUST_ROOT) --- The path to a checkout of the [`rust-lang/rust`](https://github.com/rust-lang/rust) GitHub repository. This is used for test linking.